In recent years, as part of energy-saving measures, an electric-generating system using natural energy such as, for example, wind power or wave power, has attracted attention. Moreover, there has been increasing development of recovery systems for regenerative power obtained by drive of a moving body such as an automobile, a train, or a construction machine. In order to effectively use electric energy obtained by the aforementioned systems, an electric energy storage device for storing the electric energy therein is required. For such a purpose, various types of electric energy storage devices using a secondary battery or an electric double-layer capacitor have been proposed. For example, in Patent Literature 1, an electric energy storage device in which an electric double-layer capacitor is connected in parallel to a lithium secondary battery is proposed.